Bulgaria on July 26 insisted that Russian aircraft had violated international rules while flying over the Black Sea although it conceded they had not flown into Bulgarian airspace.

The Bulgarian Ministry of Defense said Russian aircraft turned off their transponders — communication devices that allow an airplane to be located — and flew over the sea between the two nations without previously announcing their flight schedules.

Bulgaria, a NATO member, was reacting to a Russia's Defense Ministry statement on July 25 denying any such violations and insisting that Russian aircraft follow international rules and always kept their transponders on.

The exchange of accusations started on July 24 when Bulgarian Defense Minister Nikolay Nenchev told Nova TV that there had been a rise in violations of Bulgarian airspace by Russian military and commercial aircraft.

Nenchev said Russian military aircraft had violated "Bulgaria's area of responsibility" in NATO airspace four times in the past month.

"It is very worrying, so we have to take preventive measures" scrambling Bulgarian fighter jets, he said.

Bulgaria, a nation of 7.2 million and once the staunchest Soviet ally, joined NATO in 2004.